Molybdenum and tungsten are the only second and third row transition elements that have known biological functions. At present, over 50 molybdenum enzymes are known, and several are of clinical significance (xanthine and retinaldehyde oxidoreductases, sulfite oxidase) for human health. Fatal simultaneous deficiencies in the activities of these enzymes due to an inborn deficiency of a "molybdenum cofactor" have been well documented in children. Point defects in the sulfite oxidase (SO) protein can also produce the neurological symptoms of sulfite oxidase deficiency. These symptoms include dislocated ocular lenses, mental retardation, and, in severe cases, attenuated growth of the brain and early death. The research proposed here addresses the fundamental properties of sulfite oxidase and other molybdoenzymes by an integrated program of biophysical, biochemical and model compound studies. Variable frequency pulsed electron paramagnetic resonance (EPR) techniques, especially electron spin echo envelope modulation (ESEEM), pulsed electron-nuclear double resonance (ENDOR), and electron-electron double resonance (ELDOR) spectroscopies, will be used to probe in detail the surroundings of the molybdenum active site in wild-type and pathogenic mutants of SO through labeling with deuterium, 17O and 33S, and by complementary theoretical calculations. Recombinant mouse SO, which is highly homologous to human SO, will be used to generate a library of physiological and non- physiological mutants of mammalian SO for systematic structural, chemical, mechanistic and spectroscopic studies. The rates of intramolecular electron transfer between the molybdenum and iron centers of SO will be investigated by flash photolysis and theoretical modeling.